ISO/IEC 9834-8:2014

INTERNATIONAL ISO/IEC
STANDARD 9834-8
Third edition
2014-08-15
Information technology — Procedures for
the operation of object identifier
registration authorities —
Part 8:
Generation of universally unique
identifiers (UUIDs) and their use in object
identifiers
Technologies de l'information — Procédures opérationnelles pour les
organismes d'enregistrement d'identificateur d'objet —
Partie 8: Génération des identificateurs uniques universels (UUID) et
utilisation de ces identificateurs dans les composants d'identificateurs
d'objets
Reference number
ISO/IEC 9834-8:2014(E)
©
ISO/IEC 2014
©  ISO/IEC 2014
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ii © ISO/IEC 2014 – All rights reserved

Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are members of
ISO or IEC participate in the development of International Standards through technical committees
established by the respective organization to deal with particular fields of technical activity. ISO and IEC
technical committees collaborate in fields of mutual interest. Other international organizations, governmental
and non-governmental, in liaison with ISO and IEC, also take part in the work. In the field of information
technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft International
Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as
an International Standard requires approval by at least 75 % of the national bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
ISO/IEC 9834-8 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 6, Telecommunications and information exchange between systems, in collaboration with
ITU-T. The identical text is published as Rec. ITU-T X.667 (10/2012).
This third edition cancels and replaces the second edition (ISO/IEC 9834-8:2008), which has been technically
revised.
ISO/IEC 9834 consists of the following parts, under the general title Information technology — Procedures for
the operation of object identifier registration authorities:
— Part 1: General procedures and top arcs of the international object identifier tree
— Part 2: Registration procedures for OSI document types
— Part 3: Registration of Object Identifier arcs beneath the top-level arc jointly administered by ISO and
ITU-T
— Part 4: Register of VTE Profiles
— Part 5: Register of VT Control Object Definitions
— Part 6: Registration of application processes and application entities
— Part 7: Joint ISO and ITU-T Registration of International Organizations
— Part 8: Generation of universally unique identifiers (UUIDs) and their use in object identifiers
— Part 9: Registration of object identifier arcs for applications and services using tag-based identification

© ISO/IEC 2014 – All rights reserved iii

CONTENTS
Page
1 Scope . 1
2 Normative references . 1
2.1 Identical Recommendations | International Standards . 1
2.2 Other normative references . 1
3 Terms and definitions . 2
3.1 ASN.1 notation . 2
3.2 Registration authorities . 2
3.3 Network terms . 2
3.4 Additional definitions . 2
4 Abbreviations . 3
5 Notation . 3
6 UUID structure and representation . 3
6.1 UUID field structure . 3
6.2 Binary representation . 4
6.3 Representation as a single integer value . 4
6.4 Hexadecimal representation . 4
6.5 Formal syntax of the hexadecimal representation . 4
7 Use of a UUID as the primary integer value and Unicode label of a Joint UUID arc . 5
8 Use of a UUID to form a URN . 5
9 Rules for comparison and ordering of UUIDs . 5
10 Validation . 6
11 Variant bits . 6
12 Use of UUID fields and transmission byte order . 6
12.1 General . 6
12.2 Version . 7
12.3 Time . 7
12.4 Clock sequence . 8
12.5 Node . 8
13 Setting the fields of a time-based UUID. 9
14 Setting the fields of a name-based UUID . 9
15 Setting the fields of a random-number-based UUID . 10
Annex A – Algorithms for the efficient generation of time-based UUIDs . 11
A.1 Basic algorithm . 11
A.2 Reading stable storage . 11
A.3 System clock resolution . 11
A.4 Writing stable storage . 12
A.5 Sharing state across processes . 12
Annex B – Properties of name-based UUIDs . 13
Annex C – Generation of random numbers in a system . 14
Annex D – Sample implementation . 15
D.1 Files provided . 15
D.2 The copyrt.h file . 15
D.3 The uuid.h file . 15
D.4 The uuid.c file . 16
D.5 The sysdep.h file . 19
Rec. ITU-T X.667 (10/2012) iii

Page
D.6 The sysdep.c file . 19
D.7 The utest.c file . 21
D.8 Sample output of utest . 21
D.9 Some name space IDs . 22
Bibliography . 23

iv Rec. ITU-T X.667 (10/2012)
Introduction
This Recommendation | International Standard standardizes the generation of universally unique identifiers (UUIDs).
UUIDs are an octet string of 16 octets (128 bits). The 16 octets can be interpreted as an unsigned integer encoding, and
the resulting integer value can be used as the primary integer value (defining an integer-valued Unicode label) for an arc
of the International Object Identifier tree under the Joint UUID arc. This enables users to generate object identifier and
OID internationalized resource identifier names without any registration procedure.
UUIDs are also known as globally unique identifiers (GUIDs), but this term is not used in this Recommendation |
International Standard. UUIDs were originally used in the network computing system (NCS) [7] and later in the Open
Software Foundation's Distributed Computing Environment (DCE) [6]. ISO/IEC 11578 [5] contains a short definition
of some (but not all) of the UUID formats specified in this Recommendation | International Standard. The specification
in this Recommendation | International Standard is consistent with all these earlier specifications.
UUIDs forming a component of an OID are represented in ASN.1 value notation as the decimal representation of their
integer value, but for all other display purposes it is more usual to represent them with hexadecimal digits with a hyphen
separating the different fields within the 16-octet UUID. This representation is defined in this Recommendation |
International Standard.
If generated according to one of the mechanisms defined in this Recommendation | International Standard, a UUID is
either guaranteed to be different from all other UUIDs generated before 3603 A.D., or is extremely likely to be different
(depending on the mechanism chosen).
No centralized authority is required to administer UUIDs. Centrally generated UUIDs are guaranteed to be different
from all other UUIDs centrally generated.
A UUID can be used for multiple purposes, from tagging objects with an extremely short lifetime, to reliably
identifying very persistent objects across a network, particularly (but not necessarily) as part of an object identifier or
OID internationalized resource identifier value, or in a uniform resource name (URN).
The UUID generation algorithm specified in this Recommendation | International Standard supports very high
allocation rates: 10 million per second per machine if necessary, so UUIDs can also be used as transaction IDs. An
informative annex provides a program in the C language that will generate UUIDs in accordance with this
Recommendation | International Standard.
Three algorithms are specified for the generation of unique UUIDs, using different mechanisms to ensure uniqueness.
These produce different versions of a UUID.
The first (and most common) mechanism produces the so-called time-based version. These UUIDs can be generated at
the rate of 10 million per second. For UUIDs generated within a single computer system, a 60-bit time-stamp (used as a
Clock value) with a granularity of 100 nanoseconds, based on coordinated universal time (UTC) is used to guarantee
uniqueness over a period of approximately 1600 years. For UUIDs generated with the same time-stamp by different
systems, uniqueness is obtained by use of 48-bit media access control (MAC) addresses, specified in ISO/IEC 8802-3
(this is used as a Node value). (These addresses are usually already available on most networked systems, but are
otherwise obtainable from the IEEE Registration Authority for MAC addresses – see [4].) Alternative ways of
generating Clock and Node values are specified for the time-based version if UTC time is not available on a system, or
if there is no MAC address available.
The second mechanism produces a single UUID that is a name-based version, where cryptographic hashing is used to
produce the 128-bit UUID value from a globally unambiguous (text) name.
The third mechanism uses pseudo-random or truly random number generation to produce most of the bits in the 128-bit
value.
Clause 5 specifies the notation used for octet-order and bit-order naming, and for specification of transmission order.
Clause 6 specifies the structure of a UUID and the representation of it in binary, hexadecimal, or as a single integer
value.
Clauses 7 and 8 specify the use of a UUID in an OID or a URN respectively.
Clause 9 specifies rules for comparing UUIDs to test for equality or to provide an ordering relation between
two UUIDs.
Clause 10 discusses the possibility of checking the validity of a UUID. In general, UUIDs have little redundancy, and
there is little scope for checking their validity.
Rec. ITU-T X.667 (10/2012) v
Clause 11 describes the historical use of some bits in the UUID to define different variants of the UUID format, and
specifies the value of these bits for UUIDs defined in accordance with this Recommendation | International Standard.
Clause 12 specifies the use of the fields of a UUID in the different versions that are defined (time-based, name-based,
and random-number based versions). It also defines the transmission byte order.
Clause 13 specifies the setting of the fields of a time-based UUID.
Clause 14 specifies the setting of the fields of a name-based UUID.
Clause 15 specifies the setting of the fields of a random-number-based UUID.
All annexes are informative.
Annex A describes various algorithms for the efficient generation of time-based UUIDs.
Annex B discusses the properties that a name-based UUID should have, affecting the selection of name spaces for use
in generating such UUIDs.
Annex C provides guidance on mechanisms that can be used to generate random numbers in a computer system.
Annex D contains a complete program in the C programming language that can be used to generate UUIDs.

vi Rec. ITU-T X.667 (10/2012)
INTERNATIONAL STANDARD
RECOMMENDATION ITU-T
Information technology – Procedures for the operation of object identifier registration
authorities: Generation of universally unique identifiers and their use in object identifiers
1 Scope
This Recommendation | International Standard specifies the format and generation rules that enable users to produce
128-bit identifiers that are either guaranteed to be globally unique, or are globally unique with a high probability.
The universally unique identifiers (UUIDs) generated in conformance with this Recommendation | International
Standard are suitable either for transient use, with generation of a new UUID every 100 nanoseconds, or as persistent
identifiers.
This Recommendation | International Standard is derived from earlier non-standard specifications of UUIDs and their
generation, and is technically identical to those earlier specifications.
This Recommendation | International Standard also specifies and allows the use of UUIDs as primary values (which
define Unicode labels) for arcs beneath the Joint UUID arc. This enables users to generate and use such arcs without
any registration procedures.
This Recommendation | International Standard also specifies and allows the use of UUIDs to form a uniform resource
name (URN).
2 Normative references
The following Recommendations and International Standards contain provisions which, through reference in this text,
constitute provisions of this Recommendation | International Standard. At the time of publication, the editions indicated
were valid. All Recommendations and Standards are subject to revision, and parties to agreements based on this
Recommendation | International Standard are encouraged to investigate the possibility of applying the most recent
edition of the Recommendations and Standards listed below. Members of IEC and ISO maintain registers of currently
valid International Standards. The Telecommunication Standardization Bureau of the ITU maintains a list of currently
valid ITU-T Recommendations.
2.1 Identical Recommendations | International Standards
– Recommendation ITU-T X.660 (2011) | ISO/IEC 9834-1:2012, Information technology – Procedures for
the operation of object identifier registration authorities: General procedures and top arcs of the
international object identifier tree.
– Recommendation ITU-T X.680 (2008) | ISO/IEC 8824-1:2008, Information technology – Abstract
Syntax Notation One (ASN.1): Specification of basic notation.
2.2 Other normative references
– ISO/IEC 8802-3:2000, Information technology – Telecommunications and information exchange
between systems – Local and metropolitan area networks – Specific requirements – Part 3: Carrier
sense multiple access with collision detection (CSMA/CD) access method and physical layer
specifications.
– ISO/IEC 10118-3:2004, Information technology – Security techniques – Hash functions – Part 3:
Dedicated hash-functions.
– ISO/IEC 10646:2012, Information technology – Universal Coded Character Set (UCS).
– IETF RFC 1321 (1992), The MD5 Message-Digest Algorithm.
– IETF RFC 2141 (1997), URN Syntax.
– FIPS PUB 180-3:2008, Federal Information Processing Standards Publication, Secure Hash Standard
(SHS).
Rec. ITU-T X.667 (10/2012) 1
3 Terms and definitions
For the purposes of this Recommendation | International Standard, the following definitions apply.
3.1 ASN.1 notation
This Recommendation | International Standard uses the following terms defined in Rec. ITU-T X.680 |
ISO/IEC 8824-1:
a) Coordinated universal time (UTC);
b) object identifier type;
c) OID internationalized resource identifier type.
3.2 Registration authorities
This Recommendation | International Standard uses the following terms defined in Rec. ITU-T X.660 |
ISO/IEC 9834-1:
a) International Object Identifier tree;
b) IRI/URI value;
c) OID internationalized resource identifier;
d) object identifier;
e) registration;²
f) registration authority;
g) registration procedures;
h) secondary identifier;
i) Unicode character;
j) Unicode label.
3.3 Network terms
This Recommendation | International Standard uses the following term defined in ISO/IEC 8802-3:
– MAC address.
3.4 Additional definitions
3.4.1 cryptographic-quality random-number: A random number or pseudo-random number generated by a
mechanism, which ensures sufficient spread of repeatedly-generated values to be acceptable for use in cryptographic
work (and is used in such work).
3.4.2 Joint UUID arc: An arc beneath the node of the International Object Identifier tree identified by the ASN.1
{joint-iso-itu-t(2) uuid(25)} and the ASN.1 OID internationalized resource identifier
object identifier value
"/UUID".
value
3.4.3 name-based version: A UUID that is generated using cryptographic hashing of a name space name and a
name in that name space.
3.4.4 name space: A system for generating names of objects that ensures unambiguous identification within that
name space.
NOTE – Examples of name spaces are the network domain name system, URNs, OIDs, Directory distinguished names (see [1]),
and reserved words in a programming language.
3.4.5 random-number-based version: A UUID that is generated using a random or pseudo-random number.
3.4.6 standard UUID variant: The variant of the possible UUID formats that is specified by this Recommendation
| International Standard.
NOTE – Historically, there have been other specifications of UUID formats that differ from the variant specified in this
Recommendation | International Standard. UUIDs generated according to all these variant formats are all distinct.
3.4.7 time-based version: A UUID in which uniqueness is obtained by the use of a MAC address to identify a
system, and a Clock value based on the current UTC time.
2 Rec. ITU-T X.667 (10/2012)
3.4.8 universally unique identifier (UUID): A 128-bit value generated in accordance with this Recommendation |
International Standard, or in accordance with some historical specifications, and providing unique values between
systems and over time (see also 3.4.6).
4 Abbreviations
For the purposes of this Recommendation | International Standard, the following abbreviations apply:
ASN.1 Abstract Syntax Notation One
GUID Globally Unique Identifier
MAC Media Access Control
MD5 Message Digest algorithm 5
OID Object Identifier
OID-IRI OID internationalized resource identifier
SHA-1 Secure Hash Algorithm 1
URL Uniform Resource Locator
URN Uniform Resource Name
UTC Coordinated Universal Time
UUID Universally Unique Identifier
5 Notation
5.1 This Recommendation | International Standard specifies a sequence of octets for a UUID using the terms first
and last. The first octet is also called "octet 15" and the last octet "octet 0".
5.2 The bits within a UUID are also numbered as "bit 127" to "bit 0", with bit 127 as the most significant bit of
octet 15 and bit 0 as the least significant bit of octet 0.
5.3 When figures and tables are used in this Recommendation | International Standard, the most significant octet
(and the most significant bit) are displayed on the left of the page. This corresponds with a transmission order of octets
in which the left-most octets are transmitted first.
5.4 A number of values used in this Specification are expressed as the value of an unsigned integer of a given
bit-length (N say). The bits of the N-bit unsigned integer value are numbered "bit N-1" to "bit 0", with bit N-1 as the
most significant bit and bit 0 as the least significant bit.
5.5 These notations are used solely for the purposes of this Specification. Representations in computer memory
are not standardized, and depend on the system architecture.
6 UUID structure and representation
6.1 UUID field structure
6.1.1 A UUID is specified as an ordered sequence of six fields. A UUID is specified in terms of the concatenation
of these UUID fields. The UUID fields are named:
a) the "TimeLow" field;
b) the "TimeMid" field;
c) the "VersionAndTimeHigh" field;
d) the "VariantAndClockSeqHigh" field;
e) the "ClockSeqLow" field;
f) the "Node" field.
6.1.2 The UUID fields are defined to have a significance in the order listed above, with "TimeLow" as the most
significant field (bit 31 of "TimeLow" is bit 127 of the UUID), and "Node" as the least significant field (bit 0 of "Node"
is bit 0 of the UUID).
Rec. ITU-T X.667 (10/2012) 3
6.1.3 The contents of these UUID fields are specified in terms of a Version, Variant, Time, Clock Sequence, and
Node unsigned integer value (each with a fixed bit-size). The setting of these values is specified in clause 12 and their
mapping to the above UUID fields is specified in 12.1.
NOTE – As part of the names of some of the UUID fields (for example, TimeLow, TimeMid, and TimeHigh) imply, the
sequential order of the bits in a UUID (bit 127 to bit 0) that derive from a particular unsigned integer value (for example, from
bits 59 to 0 of the Time value) is not the same as the sequential order of the bits in that unsigned integer value. This is for
historical reasons.
6.2 Binary representation
6.2.1 A UUID shall be represented in binary as 16 octets formed by the concatenation of the unsigned integer fixed-
length encoding of each of its fields into one or more octets. The number of octets to be used for each field shall be:
a) the "TimeLow" field: four octets;
b) the "TimeMid" field: two octets;
c) the "VersionAndTimeHigh" field: two octets;
d) the "VariantAndClockSeqHigh" field: one octet;
e) the "ClockSeqLow" field: one octet;
f) the "Node" field: six octets.
NOTE – This order of UUID fields is the usual representation within a computer system, and in the hexadecimal text
representation (see 6.4).
6.2.2 The most significant bit of the unsigned integer encoding of each UUID field shall be the most significant bit
of its first octet (octet N, the most significant octet), and the least significant bit of the unsigned integer encoding shall
be the least significant bit of its last octet (octet 0, the least significant bit).
6.2.3 The UUID fields shall be concatenated in the order of their significance (see 6.1.2) with the most significant
field first and the least significant field last.
6.3 Representation as a single integer value
A UUID can be represented as a single integer value. To obtain the single integer value of the UUID, the 16 octets of
the binary representation shall be treated as an unsigned integer encoding with the most significant bit of the integer
encoding as the most significant bit (bit 7) of the first of the sixteen octets (octet 15) and the least significant bit as the
least significant bit (bit 0) of the last of the sixteen octets (octet 0).
NOTE – The single integer value is used when the UUID forms the primary integer value of a Joint UUID arc as specified in
clause 7.
6.4 Hexadecimal representation
For the hexadecimal format, the octets of the binary format shall be represented by a string of hexadecimal digits, using
two hexadecimal digits for each octet of the binary format, the first being the value of the four high-order bits of
octet 15, the second being the value of the four low-order bits of octet 15, and so on, with the last being the value of the
low-order bits of octet 0 (see 6.5). A HYPHEN-MINUS (45) character (see ISO/IEC 10646) shall be inserted between
the hexadecimal representations of each pair of adjacent fields, except between the "VariantAndClockSeqHigh" field
and the "ClockSeqLow" field (see the example in clause 8).
6.5 Formal syntax of the hexadecimal representation
6.5.1 The formal definition of the UUID hexadecimal representation syntax is specified using the extended BNF
notation defined in Rec. ITU-T X.680 | ISO/IEC 8824-1, clause 5, except that there shall be no white-space between the
lexical items.
6.5.2 The "hexdigit" lexical item is used in the BNF specification and is defined as follows:
Name of lexical item – hexdigit
A "hexdigit" shall consist of exactly one of the characters:
A B C D E F a b c d e f 0 1 2 3 4 5 6 7 8 9
4 Rec. ITU-T X.667 (10/2012)
6.5.3 The hexadecimal representation of a UUID shall be the production "UUID":
UUID ::=
TimeLow
"-" TimeMid
"-" VersionAndTimeHigh
"-" VariantAndClockSeqHigh ClockSeqLow
"-" Node
TimeLow ::=
HexOctet HexOctet HexOctet HexOctet
TimeMid ::=
HexOctet HexOctet
VersionAndTimeHigh ::=
HexOctet HexOctet
VariantAndClockSeqHigh ::=
HexOctet
ClockSeqLow ::=
HexOctet
Node ::=
HexOctet HexOctet HexOctet HexOctet HexOctet HexOctet
HexOctet ::=
hexdigit hexdigit
6.5.4 Software generating the hexadecimal representation of a UUID shall not use upper case letters.
NOTE – It is recommended that the hexadecimal representation used in all human-readable formats be restricted to lower-case
letters. Software processing this representation is, however, required to accept both upper and lower case letters as specified
in 6.5.2.
7 Use of a UUID as the primary integer value and Unicode label of a Joint UUID arc
7.1 A UUID can be used as the primary integer value of a Joint UUID arc using the single integer value of the
UUID specified in 6.3.
7.2 The hexadecimal representation of the UUID defined in 6.4 can also be used as a non-integer Unicode label
for the arc.
EXAMPLE – The following is an example of the use of a UUID to form an IRI/URI value:
"oid:/UUID/f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
8 Use of a UUID to form a URN
A URN (see IETF RFC 2141) formed using a UUID shall be the string "urn:uuid:" followed by the hexadecimal
representation of a UUID defined in 6.4.
EXAMPLE – The following is an example of the string representation of a UUID as a URN:
urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6
NOTE – An alternative URN format (see [2]) is available, but is not recommended for URNs generated using UUIDs. This
alternative format uses the single integer value of the UUID specified in 6.3, and represents the above example as
"urn:oid:2.25.329800735698586629295641978511506172918".
9 Rules for comparison and ordering of UUIDs
9.1 To compare a pair of UUIDs, the values of the corresponding fields (see 6.1) of each UUID are compared, in
order of significance (see 6.1.2). Two UUIDs are equal if and only if all the corresponding fields are equal.
NOTE 1 – This algorithm for comparing two UUIDs is equivalent to the comparison of the values of the single integer
representations specified in 6.3.
Rec. ITU-T X.667 (10/2012) 5
NOTE 2 – This comparison uses the physical fields specified in 6.1.1 not the values listed in 6.1.3 and specified in clause 12
(Time, Clock Sequence, Variant, Version, and Node).
9.2 A UUID is considered greater than another UUID if it has a larger value for the most significant field in
which they differ.
9.3 In a lexicographical ordering of the hexadecimal representation of UUIDs (see 6.4), a larger UUID shall
follow a smaller UUID.
10 Validation
Apart from determining if the variant bits are set correctly, and that the Time value used in a time-based UUID is in the
future (and therefore not yet assignable), there is no mechanism for determining if a UUID is valid in any real sense, as
all possible values can otherwise occur.
11 Variant bits
11.1 The variant bits are the most significant three bits (bits 7, 6, and 5) of octet 7, which is the most significant
octet of the "VariantAndClockSeqHigh" field.
11.2 All UUIDs conforming to this Recommendation | International Standard shall have variant bits with bit 7 of
octet 7 set to 1 and bit 6 of octet 7 set to 0. Bit 5 of octet 7 is the most significant bit of the Clock Sequence and shall be
set in accordance with 12.4.
NOTE – Bit 5 is listed here as a variant bit because its value distinguishes historical formats. Strictly speaking, it is not part of the
variant value for this Recommendation | International Standard, which uses only two bits for the variant.
11.3 Table 1 lists, for information, the use of other values of the variant bits.
Table 1 – Use of the variant bits
Bit 7 Bit 6 Bit 5 Description
0 – – Reserved to provide NCS backward compatibility
1 0 – The variant specified in this Recommendation | International Standard
1 1 0 Reserved to provide Microsoft Corporation backward compatibility
1 1 1 Reserved for future use by this Recommendation | International Standard

12 Use of UUID fields and transmission byte order
12.1 General
12.1.1 Table 2 gives the position and summarizes the use of the various UUID fields in the binary representation.
Table 2 – Position and use of UUID fields
Field Octet # in UUID Description
"TimeLow" 15-12 The low-order bits of the Time value (32 bits)
"TimeMid" 11-10 The middle bits of the Time value (16 bits)
The Version (4 bits) followed by the high-order bits of the Time
"VersionAndTimeHigh" 9-8
value (12 bits)
"VariantAndClockSeqHigh" 7 The Variant bits (2 bits) followed by the high-order bits of the
Clock Sequence (6 bits)
"ClockSeqLow" 6 The low-order bits of the Clock Sequence (8 bits)
"Node" 5-0 The Node (see 12.5) (48 bits)

6 Rec. ITU-T X.667 (10/2012)
12.1.2 The position of the UUID fields in the binary representation is illustrated in Figure 1.

Octets 15, 11, 7, 3 Octets 14, 10, 6, 2 Octets 13, 9, 5, 1 Octets 12, 8, 4, 0

7              0 7              0 7              0 7              0
First octet
→ "TimeLow"
(Octet 15)
"TimeMid" "VersionAndTimeHigh"
"VariantAndClockSeq "ClockSeqLow" "Node" (first two octets)
High"
←
Last octet
"Node" (last four octets) (Octet 0)
Figure 1 – Position of UUID fields in the binary representation
12.1.3 It is recommended that the binary representation be used for transmission over a communication mechanism,
with the sixteen octets of the binary representation transmitted as a contiguous set of sixteen bits with the first octet
(octet 15) preceding the last octet (octet 0) in the transmission.
NOTE 1 – The order of the bits within an octet is determined by the communication mechanism specification.
NOTE 2 – The use of sixteen consecutive octets for transmission of a UUID, in the order specified above, is recommended, but
protocol specifications may choose alternative means of transferring a UUID, including fragmentation or transmission of only
portions of the UUID (such as the parts that contribute to the Time value).
12.2 Version
12.2.1 The three alternative means of generating a UUID (time-based, name-based, and random-number-based) are
identified and distinguished by the most significant four bits of the "VersionAndTimeHigh" field (bits 7 to 4 of octet 9
of the UUID). UUIDs generated using these different mechanisms are called "different UUID versions".
NOTE – Describing this as a "different UUID version" is slightly misleading, but the name is used for historical reasons. There is
no concept of a traditional "version number" for UUID formats, where new versions may be defined as a revision of this
Recommendation | International Standard. Any new UUID format needed in the future would be identified by a different value of
the variant bits.
12.2.2 Table 3 lists currently defined "UUID versions", using the first four bits of the "VersionAndTimeHigh" field
(bits 7 to 4 of octet 9 of the UUID). It also assigns an integer "Version" value to each combination of bits.
NOTE – A version value of 2 is not used, for compatibility with historical definitions of the UUID. Version values of 0 and 6 to
15 are reserved for future use.
Table 3 – Currently defined UUID versions
Version
Bit 7 Bit 6 Bit 5 Bit 4 Description
value
0 0 0 1 1 The time-based version specified in this Recommendation | International
Standard (see clause 13)
0 0 1 0 2 Reserved for DCE Security version, with embedded POSIX UUIDs
0 0 1 1 3 The name-based version specified in this Recommendation | International
Standard with MD5 hash (see clause 14)
0 1 0 0 4 The random-number-based version specified in this
Recommendation | International Standard (see clause 15)
0 1 0 1 5 The name-based version specified in this Recommendation | International
Standard with SHA-1 hash (see clause 14)
12.3 Time
12.3.1 Time shall be a 60-bit value.
NOTE – The name "Time" is appropriate for the time-based version of a UUID (version 1), but is also used for the contents of
the corresponding value in the other versions of a UUID (versions 3 and 4).
12.3.2 For the time-based version of a UUID, Time shall be a count of the 100 nanosecond intervals of coordinated
universal time (UTC) since the midnight at the start of the 15 October 1582 (the date of the Gregorian reform to the
Christian calendar).
Rec. ITU-T X.667 (10/2012) 7
NOTE 1 – Before the establishment of the Bureau International de l'Heure (International Time Bureau), every minute contained
precisely sixty seconds. Since then, leap seconds have occurred when necessary, increasing (or potentially reducing) the number
of seconds per year.
NOTE 2 – Portable systems may have problems determining UTC time, as they are often locked into the local time of their home
base. Provided that they continue using the local time of their home base, or change the Clock Sequence value (see 12.4), the
UUIDs that they generate will still be unique.
NOTE 3 – For systems that do not have access to broadcast time signals, a system clock recording local time can be used with a
time differential added, provided that no UUIDs are generated in the period when a change from daylight saving time occurs, or a
change in the Clock Sequence value (see 12.4) is made.
12.3.3 For the name-based version of a UUID, this shall be a 60-bit value constructed from a globally unique name
as specified in clause 14.
NOTE – Examples of a globally unique name are OIDs, URNs, and Directory distinguished names (see [1]).
12.3.4 For the random-number-based version of a UUID, this shall be a randomly or pseudo-randomly generated
60-bit value as specified in clause 15.
12.4 Clock sequence
12.4.1 For the time-based version of the UUID, the Clock Sequence is used to help avoid duplicates that could arise
when the value of Time is set backwards or if the Node value is changed.
NOTE – The name "Clock Sequence" is appropriate for the time-based version of a UUID, but is also used for the contents of the
corresponding value in the name-based and random-number-based versions of a UUID.
12.4.2 If the Time value is set backwards, or might have been set backwards (for example, while the system was
powered off), then the UUID generator cannot know whether a UUID has already been generated with Time values
larger than the value to which the Time is now set. In such situations, the Clock Sequence value shall be changed.
NOTE – If the previous value of the Clock Sequence is known, it can be just incremented; otherwise it should be set to a
cryptographic-quality random or pseudo-random value.
12.4.3 Similarly, if the Node value changes (for example, because a network card has been moved between
machines), the Clock Sequence value shall be changed.
12.4.4 The Clock Sequence shall be originally (that is, once in the lifetime of a system producing UUIDs) initialized
to a random number that is not derived from the Node value.
NOTE – This is in order to minimize the correlation across systems, providing maximum protection against MAC addresses that
may move or switch from system to system rapidly.
12.4.5 For the name-based version of the UUID, the Clock Sequence shall be a 14-bit value constructed from a name
as specified in clause 14.
12.4.6 For the random-number-based version of the UUID, the Clock Sequence shall be a randomly or pseudo-
randomly generated 14-bit value as specified in clause 15.
12.5 Node
12.5.1 For the time-based version of a UUID, the Node value shall consist of a MAC address (see ISO/IEC 8802-3),
usually the host address of some network interface.
12.5.2 For systems with multiple MAC addresses, any available address can be used except a multicast address.
Octet 5 of the UUID (the first octet of the "Node") shall be set to the first octet of the MAC address that is transmitted
by an ISO/IEC 8802-3-conformant system.
NOTE 1 – This octet contains the global/local bit and the unicast/multicast bit. It is required that the unicast/multicast bit be set to
unicast in order to avoid clashes with addresses generated in accordance with 12.5.3.
NOTE 2 – It is possible to obtain a block of MAC addresses from the MAC address registration authority (see [4]).
12.5.3 For systems with no MAC address, a cryptographic-quality random or pseudo-random number may be used
(see Annex C). The multicast bit shall be set in such addresses.
NOTE – This is to ensure that the generated addresses never conflict with addresses obtained from network cards as specified
in 12.5.2.
12.5.4 For a name-based UUID, the Node value shall be a 48-bit value constructed by canonicalization and hashing
from a globally unique name as specified in clause 14.
12.5.5 For a random-number-based UUID, the Node value shall be a randomly or pseudo-randomly generated 48-bit
value as specified in clause 15.
8 Rec. ITU-T X.667 (10/2012)
13 Setting the fields of a time-based UUID
The fields of a time-based UUID shall be set as follows:
– Determine the values for the UTC-based Time and the Clock Sequence to be used in the UUID, as
specified in 12.3 and 12.4.
– For the purposes of this algorithm, consider Time to be a 60-bit unsigned integer and the Clock Sequence
to be a 14-bit unsigned integer. Sequentially number the bits in each value, with zero for the least
significant bit.
– Set the "TimeLow" field equal to the least significant 32 bits (bits 31 through 0) of Time in the same
order of significance.
– Set the "TimeMid" field equal to bits 47 through 32 from the Time in the same order of significance.
– Set the 12 least significant bits (bits 11 through 0) of the "VersionAndTimeHigh" field equal to bits 59
through 48 from Time in the same orde
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